1. Field of the Invention
The invention relates to an arrangement for cumulative summing and compressing digital data and in particular to such an arrangement which operates in conjunction with a computer.
2. State of the Prior Art
In data processing it often happens that operations are performed on non-significant items of data in an overall quantity of data which may be large. The result is frequently a considerable loss of time and a useless immobilisation of circuits and storage members such as the computer memories. In cases where data is remotely transmitted, it also frequently happens that the transmitted signals which represent this data are redundant too. For these reasons, existing arrangements make it possible, by compress ing the data before it is to be transmitted along a channel in a relatively short space of time, or else they allow a channel having a narrower pass band to be used so that the significant data is transmitted, thus removing the causes of redundancy to the maximum extent. Of known data compression arrangements, some are designed to analyse the word positions of the data to be transmitted to find non-significant word positions and replace them by a special character. Other arrangements are designed to find the significant positions in a data item and their positions in the grouping formed by the data item. Variable-length cyclic coding techniques have also been used to compress digital data. These techniques consist in transmitting with each non-redundant sample a binary image of the number or length of the redundant samples (which are not transmitted), which have appeared since the transmission of the previous sample. The position in a frame of any given data sample may then be found by adding the number of redundant and non-redundant samples preceding it. Using other known arrangements which combine the predictive coding and variable length cyclic coding techniques, it is possible to attain relatively high ratios of compression, a ratio of compression being defined as the ratio between the number of characters used by the data before compression and the number of characters used after compression. Since the majority of these different arrangements makes it necessary for the zones to be compressed to be examined character by character, they operate fairly slowly with the assistance of a computer. Other arrangements operate more quickly, but the results obtained are less satisfactory and the circuits of which they are made up are relatively complex.
Taking as an example the case of a list of words which has to be consulted, sucha as a language dictionary, it is found to be possible to lay down a maximum length, in the case of the longest word in the list, and an average length which applies to the majority of the words in the list and depends on the greatest frequency of occurence. This causes a waste of space which results from following the principle of storing words in zones of a fixed maximum length. In another case, where the data to be processed represents fault symptoms which are collected in order to perform a diagnosis, it is a long and expensive process to have the result of a test analysed by a program and requires in particular a considerable amount of space in the circuits of the computer, such as its memories, and also requires the intervention of a technical specialist to locate a fault. However, generally speaking, the probability of certain components failing is low, and most of the time, an accurate diagnosis may be made with the help of a dictionary which lists a number of faults very much smaller than the number of binary combinations which result from combining the symptoms collected, in accordance with the possible values of the latter.